Constant velocity universal joints are used in power transmission systems of automobiles, aircrafts, ships, various industrial machines, and the like. The constant velocity universal joints to be incorporated into a drive shaft, a propeller shaft, and the like for transmitting a rotational force from an engine of an automobile to wheels at constant velocity are classified into such two types as a fixed type constant velocity universal joint and a plunging type constant velocity universal joint. The two types of constant velocity universal joints have a structure capable of transmitting rotation at constant velocity even when two shafts, namely, a drive shaft and a driven shaft are coupled to each other to form an operating angle.
The drive shafts for transmitting power from the engine of the automobile to driving wheels need to allow angular displacement and axial displacement along with changes in relative positional relationship between a differential and the wheels. Thus, the drive shafts each generally include a plunging type constant velocity universal joint capable of allowing the angular displacement and the axial displacement on the differential side (inboard side), a fixed type constant velocity universal joint capable of forming a high operating angle on the driving wheel side (outboard side), and a shaft for coupling the two types of constant velocity universal joints to each other.
As an example of the fixed type constant velocity universal joint, a Rzeppa constant velocity universal joint is known. As illustrated in FIG. 15, a Rzeppa constant velocity universal joint 121 mainly includes an outer joint member 122, an inner joint member 123, torque transmitting balls 124, and a cage 125. In a spherical inner peripheral surface 128 of the outer joint member 122, a plurality of track grooves 126 are formed equiangularly so as to extend along an axial direction. In a spherical outer peripheral surface 129 of the inner joint member 123, track grooves 127 opposed to the track grooves 126 of the outer joint member 122 are formed equiangularly so as to extend along the axial direction. Each of the plurality of balls 124 for transmitting torque is incorporated between the track groove 126 of the outer joint member 122 and the track groove 127 of the inner joint member 123. The cage 125 for holding the balls 124 is arranged between the spherical inner peripheral surface 128 of the outer joint member 122 and the spherical outer peripheral surface 129 of the inner joint member 123.
The spherical inner peripheral surface 128 of the outer joint member 122 and the spherical outer peripheral surface 129 of the inner joint member 123 each have a curvature center formed at a joint center O. Further, a spherical outer peripheral surface 130 and a spherical inner peripheral surface 131 of the cage 125 each have a curvature center formed at the joint center O as well. On the other hand, a curvature center A of the track groove 126 of the outer joint member 122 and a curvature center B of the track groove 127 of the inner joint member 123 are offset in the axial direction by equal distances with respect to the joint center O. Thus, when the joint forms an operating angle, the rotation is transmitted at a constant velocity between two axes of the outer joint member 122 and the inner joint member 123. A spline 136 is formed in an inner peripheral surface 135 of the inner joint member 123, and a spline 137 of a shaft 132 is fitted into the spline 136. With this configuration, the inner joint member 123 and the shaft 132 are coupled to each other so as to allow torque transmission therebetween.
In recent years, there has been an increasing demand for downsizing and light-weighting of the constant velocity universal joint as well as a demand for higher-power automobiles. Further, there has been a demand for a higher steering angle of front wheels to be achieved by increasing the operating angle of the fixed type constant velocity universal joint so as not to increase the turning radius of vehicles. What is most difficult in downsizing and light-weighting of the fixed type constant velocity universal joint 121 is to secure the strength of the fixed type constant velocity universal joint 121 at high operating angles (high-angle strength). To evaluate the high-angle strength, a quasi-static torsional test is often conducted. The quasi-static torsional test refers to a test of measuring breaking torque by applying torque while rotating the constant velocity universal joint at low velocity in consideration of actual vehicle conditions. According to the quasi-static torsional test thus conducted, the strength of the constant velocity universal joint 121 depends on the strength of a carburized component, such as the cage 125 and the inner joint member 123. In particular, the strength of the constant velocity universal joint 121 greatly depends on the strength of the cage 125. Thus, in order to achieve the downsizing and light-weighting of the fixed type constant velocity universal joint 121, there is a challenge to increase the strength of the carburized component, in particular, the strength of the cage 125.
In the following patent documents, there have been proposed various measures to increase the strength of the carburized component of the constant velocity universal joint.